Lead-free and preferably arsenic-free lanthanum heavy flint glass

ABSTRACT

The lead-free and preferably arsenic-free optical glass has a refractive index n d  of 1.84≦n d ≦1.96 and an Abbé number ν d  of 27≦ν d ≦36, with good chemical resistance, excellent crystallization resistance and the following composition (in % by weight based on oxide): SiO 2 , 1 to 8, B 2 O 3 , 13 to 19.5, La 2 O 3 , 34 to 50; MgO, 0 to 6, CaO, 0 to 6, BaO, 0 to 6, ZnO, 0 to 9 with Σ MO, 1 to 10; and TiO 2 , 4 to 15, ZrO 2 , 0 to 11, Nb 2 O 5 , 6 to 14.5. In addition, the glass according to the invention, as well as having a maximum alkali metal oxide content of 10% by weight, may also contain standard refining agents, although arsenic is not preferred. The glass according to the invention is used in imaging, projection, telecommunications, optical communication and/or laser technology.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to lead-free and preferably alsoarsenic-free lanthanum heavy flint glass with a refractive index n_(d)of 1.84≦n_(d)≦1.96 and an Abbé number ν_(d) of 27≦ν_(d)≦36, and to theuses of this glass.

[0003] 2. Description of the Related Art

[0004] Conventional optical glass in the lanthanum-heavy flint rangewith a low Abbé number for the application areas of imaging, projection,telecommunications, optical communication technology and lasertechnology generally contains PbO, in order to achieve the desiredoptical properties, i.e. a refractive index n_(d) of 1.84≦n_(d)≦1.96and, in particular, the high dispersion, i.e. a low Abbé number ν_(d) of27≦ν_(d)≦36. However, PbO reduces the chemical resistance of these typesof glass. Furthermore, materials with high chemical resistance arebecoming increasingly important for use in high added value products.Moreover, As₂O₃ is often used as a refining agent. Since in recent yearsthe glass components PbO and As₂O₃ have been regarded as environmentallyharmful, most manufacturers of optical instruments and products tendpreferably to use lead-free and arsenic-free glass.

[0005] Known lead-free glass compositions with a high refractive indexand a low Abbé number generally contain extremely large quantities ofTiO₂ in a silicate matrix, which leads to glass, which is very highlysusceptible to crystallization and extremely difficult to process.

[0006] In addition, in terms of melting technology there have recentlybeen reports of increased demand for “short” glass, i.e. for glass whoseviscosity varies extremely strongly with the temperature. During themanufacturing process, this characteristic has the advantage that thehot-shaping times, i.e. the mold-closed times, can be reduced. Thisfirstly increases throughput and secondly protects the mold material,with the result that total production costs can be reduced considerably.Also, the more rapid cooling which is thereby made possible also allowsglass with a relatively strong tendency to crystallize to be processed,i.e. correspondingly longer glass, and means that preliminarynucleation, which could cause problems in subsequent secondaryhot-forming steps, is avoided.

[0007] Therefore, a composition range for short optical glass, whichallows the desired optical properties with regard to n_(d) and ν_(d) tobe achieved, even without use of PbO and As₂O₃, and furthermore with areduced TiO₂ content, would be advantageous.

[0008] However glass with a similar optical position or comparablechemical composition, which has hitherto been disclosed in the priorart, has serious drawbacks.

[0009] For example, DE 691 356 (Eastman Kodak) describes silicate-freelanthanum borate glass for achieving a similar optical position, namelya very high refractive index with an extremely low dispersion (i.e. ahigh Abbé number), but this glass is at very great risk ofcrystallizing. Moreover, to stabilize this glass it is preferable tointroduce very expensive tantalum oxide (or optionally highly toxicthorium oxide or expensive tungsten oxide) as crystallization inhibitor.Furthermore, when SiO₂ is used without the addition of alkali metaloxides, opacification phenomena have been described, and consequentlythe use of silicate is obligatorily linked to the addition of alkalimetals.

[0010] In a corresponding way to the document discussed above, JP78-004023 A (Ohara) also describes glass with an extremely lowdispersion and very high refractive index. The glass described differsfrom the glass described in DE 691 356 through the obligatoryrequirement that hafnium must be used to stabilize the lanthanum boratematrix. However, on account of the difficulty of purifying rawmaterials, this component is extremely expensive. Moreover, it has avery high melting point (melting point 3050° C.) compared to its moreusual homolog TiO₂ (melting point 1560° C.) and ZrO₂ (melting point1700° C.) with comparable physico-chemical characteristics. The resultof this is that the melting process is made considerably more difficultby additions of hafnium oxide.

[0011] JP 84-050048 A (Ohara) describes silicate-containing (>8% byweight) lanthanum borate glasses, without alkali metal oxidesnecessarily being added. Since the solubility of lanthanum oxide in aborosilicate matrix is significantly worse than in a pure borate matrix,the maximum amount of lanthanum, which can be used in this glass, islimited. Therefore, the result is either glass whose refractive indicesare lower on account of a lower La₂O₃ content or whose crystallizationresistance is adversely affected by increasing the refractive index withadditional oxides.

[0012] DE 31 38 137 (SCHOTT GLAS) relates to glass with an extremely lowdispersion combined, at the same time, with a high refractive index. Thestabilizing effect with respect to the tendency to crystallize caused bythe use of silicate without alkali metals is achieved by adding largeamounts of Nb₂O₅ (≧15% by weight). The glass described in this patentdiffers from that of DE 691 356 by the use of silicate in the absence ofalkali metals and tantalum. However, since Nb₂O₅ is a relativelyexpensive component, such high levels of Nb₂O₅ are not economical.

[0013] DE 10 47 994 (Izumitani, et al) deals with lanthanum borate glasswith a particularly high borate content (≧20% by weight). Although thishas a positive influence on the solubility of lanthanum and thereforereduces the tendency to crystallize when silicate is used at the sametime, the maximum refractive index, which can be reached, is reduced tobelow 1.87. Therefore, the glass described in this document tends to bedesigned for moderate optical applications with good chemicalresistance, high grinding hardness, i.e. good machining properties, andlow crystallization, rather than to achieve a high refractive indexcombined with a low dispersion.

SUMMARY OF THE INVENTION

[0014] Therefore, the object of the present invention is to providelead-free and preferably also arsenic-free optical glass for thefollowing application areas: imaging, projection, telecommunications,optical communication technology and/or laser technology and to provideglass with a refractive index n_(d) of 1.84≦n_(d)≦1.96 and an Abbénumber ν_(d) of 27 ≦ν_(d)≦36, which has good melting and processingproperties. Furthermore, this glass should have a good chemicalresistance and sufficient crystallization resistance, so that it can beproduced in continuously operated installations.

[0015] The above object is achieved by the embodiments of the presentinvention, which are described in the claims.

[0016] In particular, the glass of the present invention has arefractive index n_(d) of 1.84≦n_(d) ≦1.96 and an Abbé number ν_(d) of27≦ν_(d)≦36, and is characterized by the following composition (in % byweight, based on oxide): SiO₂  1 to 8, B₂O₃ 13 to 19.5, La₂O₃ 34 to 50,MgO  0 to 6, CaO  0 to 6, BaO  0 to 6, ZnO  0 to 9, TiO₂  4 to 13, ZrO₂ 0 to 11, Nb₂O₅  6 to 14.5,

[0017] wherein a sum total amount of MgO+CaO+BaO+ZnO is from 1 to 10.

[0018] According to another embodiment of the present invention, theglass has a refractive index n_(d) of 1.84≦n_(d)≦1.96 and an Abbé numberν_(d) of 27≦ν_(d)≦36, and having a composition, in percent by weightbased on oxide content, comprising: SiO₂  1 to 8, B₂O₃ 13 to 19.5, La₂O₃34 to 50, MgO  0 to 6, CaO  0 to 6, BaO  0 to 6, ZnO  0 to 9, TiO₂  4 to15, ZrO₂  0 to 11, Nb₂O₅  6 to 14.5,

[0019] wherein a sum total amount of MgO+CaO+BaO+ZnO is from 1 to 10.

[0020] According to the present invention, the details given with regardto the weight of the components are in each case based on oxide unlessstated otherwise.

[0021] The glass according to the invention has the same optical data asknown optical glass of this optical position. However, it is alsodistinguished by good chemical resistance and processing properties,lower production costs on account of reduced raw material and processcosts, crystallization resistance, which is sufficient on account of theglass being short, and by good environmental compatibility.

[0022] The glass according to the invention satisfies both therequirement for good melting and processing properties, with arefractive index n_(d) of 1.84≦n_(d)≦1.96, preferably 1.85≦n_(d)≦1.95,particularly preferably n_(d) of 1.86≦n_(d)≦1.94, and most preferablyn_(d) of 1.88≦n_(d)≦1.93, and an Abbé number ν_(d) of 27≦ν_(d)≦36,preferably 28≦ν_(d)≦35, particularly preferably 28≦ν_(d)≦34, and mostpreferably 29≦ν_(d)≦33. The glass according to the invention also hasgood chemical resistance and resistance to crystallization, as required,while at the same time does not contain PbO, and preferably also not anyAs₂O₃.

[0023] The basic glass system is a lanthanum borate glass, the boratebeing responsible for the solubility of the lanthanum. It has been foundthat stable glass is formed at La₂O₃: B₂O₃ ratios of ≦3.0. The range,which is preferred according to the invention, starts at La₂O₃:B₂O₃ ofaround, approximately 2.6. Although lower ratios are desirable withregard to resistance to crystallization, they would requiresignificantly higher absolute borate contents, with the result that itwould no longer be possible to introduce sufficient amounts of highlyrefractive components, which do not promote crystallization to achievethe refractive index position, which is desired in the present context.

[0024] To achieve the desired optical position, the glass according tothe invention contains a La₂O₃ content of at least 34 percent by weight,preferably at least 37 percent by weight, more preferably at least 39percent by weight, most preferably at least 40 percent by weight. TheLa₂O₃ content is at most 50 percent by weight, preferably at most 47percent by weight, particularly preferably at most 45 percent by weight,most preferably at most 44 percent by weight.

[0025] According to the invention, the B₂O₃ content is preferably atleast 13% by weight, preferably at least 15% by weight, more preferablyat least 16% by weight, most preferably at least 17% by weight. Theglass according to the invention contains at most 19.5% by weight,preferably at most 19% by weight of B₂O₃.

[0026] The glass according to the invention is restricted to SiO₂contents of at most 8% by weight, allowing up to 50% by weight oflanthanum oxide to be used. Therefore, there is no need to add furthercomponents, which increase the refractive index but promotecrystallization in order to achieve the high refractive index positions.

[0027] Therefore, the glass according to the invention contains onlysmall amounts of the glass former SiO₂, i.e. at most 8% by weight,preferably at most 7.5% by weight, more preferably at most 7% by weight,most preferably at most 6% by weight. However, the glass according tothe invention always contains small quantities of SiO₂, in an amount ofat least 1% by weight, preferably at least 3% by weight, particularlypreferably at least 4 % by weight. This low SiO₂ content is used toimprove the processing properties by increasing the mechanical strengthof the material. For example, good abrasion hardness and chemicalresistance can be achieved as a function of the quantity used. However,the amount of SiO₂ added must be restricted to the upper limit describedabove, since otherwise the solubility of the lanthanum in the matrix isreduced. This in turn leads to glasses which lack resistance tocrystallization or, if the La₂O₃ content has been reduced in favor ofthe SiO₂ content, to lower refractive indices and to shifts of theoverall optical position.

[0028] The most important optical component of the glass according tothe invention, i.e. the component, which has a major influence onachieving the desired optical position with a high refractive index anda low Abbé number, is Nb₂O₅. Further Nb₂O₅ stabilizes thesilicate-containing lanthanum borate glass matrix. This component ispresent in an amount of at least 6% by weight, preferably at least 8% byweight, particularly preferably at least 9% by weight, most preferablyat least 10% by weight, in the glass. This component is present in theglass according to the invention at most in an amount of 14.5 % byweight, preferably 14% by weight, and particularly preferably 13% byweight. Increasing the amount of this component to above 14.5% by weightwould lead to increased batch preparation costs. Furthermore, on accountof the glass being short, there is no need to exploit the furtherstabilizing effect in the processes. Reducing the level of thiscomponent to below 6% by weight would increase the likelihood ofcrystallization to such an extent that it would only be possible toobtain a glass with considerable difficulty.

[0029] The glass according to the invention is what is known as a “shortglass”; this expression is understood as meaning that the viscosity ofthe glass drops relatively quickly as the temperature falls or increasesrelatively quickly as the temperature rises.

[0030] This “shortness” of the glass according to the invention isachieved by the combination of a defined quantity of TiO₂ with a definedquantity of one or more alkaline-earth metal oxides.

[0031] The amount of TiO₂ in the glass according to the invention is atmost 15% by weight, preferably at most 13% by weight, particularlypreferably at most 12% by weight, most preferably 11% by weight.Increasing the TiO₂ content to over 15% by weight would firstly lead tothe viscosity-temperature profile being made undesirably extreme, i.e.the glass would become so “short” that it would be difficult to processunder standard conditions. Furthermore, such a high TiO₂ content woulddetrimentally lead to a considerable increase in the tendency of theglass to crystallize. Therefore, according to preferred embodiments,TiO₂ may if appropriate be partially replaced by ZrO₂ (0-11% by weight,preferably 1-9% by weight, particularly preferably 2-9% by weight, mostpreferably 3-8% by weight). Furthermore, TiO₂ content over 15% by weightwould result in an Abbé number, which is too low for the desiredapplication areas, i.e. in excessively high dispersion. The glasscontains at least 4% by weight, preferably at least 6% by weight,particularly preferably at least 7% by weight, most preferably 8% byweight, of this component. By contrast, reducing the content of thiscomponent to below 4% by weight would lead to there being insufficientamounts of this component to obtain a sufficiently “short” glass, andthe Abbé numbers would be too high.

[0032] According to the present invention, the glass according to theinvention also contains alkaline-earth metal oxides. The use of thealkaline-earth metal oxides in combination with the addition of TiO₂modifies the viscosity-temperature profile. Therefore, it is possible touse any alkaline-earth metal oxide individually or a mixture of two ormore alkaline-earth metal oxides within a wide range. However, the totalamount of alkaline-earth metal oxides must not exceed an amount of 10%by weight, preferably 9% by weight. To achieve the purpose of theinvention, the glass according to the invention contains at least 1% byweight, preferably at least 2% by weight, of alkaline- earth metaloxides.

[0033] However, since these components may also have an influence on theoptical position, MgO and CaO, as components with a low refractiveindex, are only present in the glass according to the invention in anamount of in each case at most 6% by weight, preferably in each case atmost 4% by weight. Therefore, particularly preferred variants of theglass according to the invention are substantially free of thesecomponents. Therefore, according to the present invention, thealkaline-earth metal oxides contained in the glass according to theinvention are preferably the components BaO and/or ZnO, which have ahigher refractive index. BaO is preferably present in an amount of atmost 6% by weight, preferably at most 4% by weight, most preferably atmost 3% by weight, in the glass according to the invention. The glassaccording to the invention preferably contains at least 1% by weight ofBaO. The glass according to the invention contains ZnO in an amount ofpreferably at most 9% by weight, for preference at most 7% by weight,most preferably at most 6% by weight, and preferably in an amount of atleast 1% by weight. It has been found that in particular ZnO appears toact as a crystallization inhibitor. It is therefore particularlypreferred to add at least one of these alkaline-earth metal components.

[0034] The glass according to the invention may also contain an additionof alkali metal oxides, which, however, do not produce any stabilizationof the glass in the small quantities, which are the maximum permitted.However, adding larger amounts would lead to reductions in therefractive index. However, alkali metal oxides of this type may benecessary for certain applications, if glass properties such as, forexample, an ion exchange capacity or slight variations to theviscosity-temperature profile towards flexible, near net shape hotforming are desired.

[0035] Therefore, in addition to the components mentioned above, theglass according to the invention may, if appropriate, contain a total ofat most 10% by weight, preferably at most 8% by weight, of alkali metaloxides, i.e. Na₂O, Li₂O, K₂O and Ca₂O.

[0036] Furthermore, standard refining agents may also be present, but itis preferable for the refining agents used not to be arsenic compounds.In particular, the following components, individually or as a mixture,may be present as refining agents (in % by weight): Sb₂O₃ 0-1, SnO 0-1,NaCl 0-1, SO₄ ⁻ 0-1, and/or F⁻ 0-1.

[0037] According to a preferred embodiment, the glass according to theinvention is preferably also substantially free of arsenic and arseniccompounds. The glass according to the invention, with a relativelymoderate optical position with regard to the dispersion, ia alsosufficiently resistant to crystallization, even when silicate isincluded (at most 8% by weight) without any addition of alkali metals.It is assumed that this is the case because of their “shortness”. It ispossible to dispense with expensive (or toxic) additions of Ta₂O₅, WO₃and/or thorium oxide; potentially inexpensive and ecologically harmlessalkaline-earth metal oxides are used to stabilize the glass according tothe invention. Therefore, the glass according to the invention ispreferably also free of Ta₂O₅, WO₃ and/or thorium oxide.

[0038] The present invention also relates to the use of the glasscompositions according to the invention in the application areas ofimaging, projection, telecommunications, optical communicationstechnology and/or laser technology.

[0039] Furthermore, the present invention relates to optical elements,which comprise a glass according to the invention. Optical elements inthis context may in particular be lenses and prisms, although they arenot restricted to these particular examples. Furthermore, the term“optical elements” also encompasses what are known as compactcomponents.

EXAMPLES

[0040] The glass according to the invention are produced in thefollowing way:

[0041] The raw materials for the oxides, preferably carbonates, nitratesand/or fluorides, are weighed out, one or more refining agents, such asfor example Sb₂O₃, is/are added and the materials are then intimatelymixed. The glass batch is melted at approx.1300° C. in a continuousmelting unit, then refined (1350° C.) and homogenized. The glass is castat a casting temperature of approximately 1220° C. and processed intothe desired dimensions. TABLE 1 MELTING EXAMPLE FOR 100 kg (calculated)of GLASS OXIDE % by weight Raw Material Amount of raw material, kg SiO₂3.0 SiO₂ 3.00 B₂O₃ 20.0 H₃BO₃ 35.53 La₂O₃ 46.7 La₂O₃ 46.70 ZnO 6.0 ZnO6.00 BaO 0.2 Ba(NO₃)₂ 0.34 BaO 3.8 BaCO₃ 4.89 TiO₂ 8.0 TiO₂ 8.00 Nb₂O₃12.0 Nb₂O₅ 12.00 Sb₂O₃ 0.3 Sb₂O₃ 0.30 Total 100.0 116.76

[0042] The properties of the glass obtained in this way are given inTable 2, Example 3.

[0043] Table 2 includes examples 1 to 7 of the glass compositionsaccording to the invention and their measured optical properties.

[0044] The exemplary glass compositions 1 to 7 (Table 2) according tothe invention have crystallization resistances and viscosity-temperatureprofiles such that further heat treatment (extrusion or repressing) ofthe glass is readily possible.

[0045] In comparative example 1, which is not covered by the compositionrange according to the invention, the desired optical position can onlybe achieved by adding a considerable amount of expensive Nb₂O₅. TABLE 2Melting Examples (in % by weight) Comparative Example 1 Example 2Example 3 Example 4 Example 5 Example 6 Example 7 Example 1 17181 1718217183 17184 17185 17187 16250 15988 SiO₂ 4.0 8.0 3.0 5.0 6.0 4.0 6.0 3.0B₂O₃ 16.0 20.0 20.0 19.0 15.0 19.0 18.0 21.0 La₂O₃ 41.0 37.0 47.0 45.039.0 44.0 41.0 50.0 Na₂O 3.0 2.0 MgO 2.0 2.0 4.0 CaO 2.0 2.0 4.0 BaO 4.04.0 2.0 4.0 2.0 ZnO 7.0 6.0 6.0 4.0 1.0 5.0 TiO₂ 13.0 6.0 8.0 10.0 8.09.0 9.0 4.0 ZrO₂ 7.0 9.0 2.0 6.0 5.0 6.0 5.0 Nb₂O₅ 10.0 14.0 12.0 8.012.0 10.0 13.0 17.0 Sb₂O₃ 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Total 100.3100.3 100.3 100.3 100.3 100.3 100.3 100.3 n_(d) 1.9448 1.9064 1.89081.8583 1.8772 1.8774 1.9144 1.9054 V_(d) 29.25 30.27 32.92 33.04 32.7533.62 31.61 33.07 P_(g,F) 0.6008 0.5845 0.5907 0.5905 0.5916 0.58820.5937 0.5888 ΔP_(g,F) (10⁻¹) 62 42 23 23 29 9 31. 6 p (g/cm³) 4.54 4.184.56 4.32 4.38 4.31 4.49 4.58 α₂₈₋₃₀₀ (10⁻⁶ * K⁻¹) 7.6 7.0 7.7 8.4 8.58.2 7.2 7.2 Tg (° C.) 640 660 636 591 616 650 645 660

[0046] The disclosure in German Patent Application 102 27 494.0-45 ofJun. 19, 2002 is incorporated here by reference. This German PatentApplication describes the invention described hereinabove and claimed inthe claims appended hereinbelow and provides the basis for a claim ofpriority for the instant invention under 35 U.S.C. 119.

[0047] While the invention has been illustrated and described asembodied in a lead-free and preferably also arsenic-free lanthanum heavyflint glass, it is not intended to be limited to the details shown,since various modifications and changes may be made without departing inany way from the spirit of the present invention.

[0048] Without further analysis, the foregoing will so fully reveal thegist of the present invention that others can, by applying currentknowledge, readily adapt it for various applications without omittingfeatures that, from the standpoint of prior art, fairly constituteessential characteristics of the generic or specific aspects of thisinvention.

[0049] What is claimed is new and is set forth in the following appendedclaims.

We claim:
 1. Lead-free optical glass with a refractive index n_(d) of1.84≦n_(d)≦1.96 and an Abbé number ν_(d) of 27≦ν_(d)≦36, and having acomposition, in percent by weight based on oxide content, comprising:SiO₂  1 to 8, B₂O₃ 13 to 19.5, La₂O₃ 34 to 50, MgO  0 to 6, CaO  0 to 6,BaO  0 to 6, ZnO  0 to 9, TiO₂  4 to 13, ZrO₂  0 to 11, Nb₂O₅  6 to14.5,

wherein a sum total amount of MgO+CaO+BaO+ZnO is from 1 to
 10. 2.Lead-free optical glass with a refractive index n_(d) of 1.85≦n_(d)≦1.95and an Abbé number ν_(d) of 28≦ν_(d)≦35, and having a composition, inpercent by weight based on oxide content, comprising: SiO₂  3 to 7.5,B₂O₃ 15 to 19.5, La₂O₃ 37 to 47, MgO  0 to 4, CaO  0 to 4, BaO  0 to 4,ZnO  0 to 7, TiO₂  6 to 13, ZrO₂  1 to 9, Nb₂O₅  8 to 14,

wherein a sum total amount of MgO+CaO+BaO+ZnO is from 2 to
 10. 3.Lead-free optical glass with a refractive index n_(d) of 1.86≦n_(d)≦1.94and an Abbé number ν_(d) of 28≦ν_(d)≦34, and having a composition, inpercent by weight based on oxide content, comprising: SiO₂  4 to 7, B₂O₃16 to 19.5, La₂O₃ 39 to 45, MgO  0 to 4, CaO  0 to 4, BaO  1 to 4, ZnO 2 to 9, TiO₂  7 to 12, ZrO₂  2 to 9, Nb₂O₅  9 to 13,

wherein a sum total amount of MgO+CaO+BaO+ZnO is from 2 to
 9. 4.Lead-free optical glass with a refractive index n_(d) of 1.88≦n_(d)≦1.93and an Abbé number ν_(d) of 29≦ν_(d)≦33, and having a composition, inpercent by weight based on oxide content, comprising: SiO₂  4 to 6, B₂O₃17 to 19, La₂O₃ 40 to 44, BaO  1 to 3, ZnO  1 to 6, TiO₂  7 to 11, ZrO₂ 3 to 8, Nb₂O₅ 10 to
 13.


5. Glass as defined in claim 1, further comprising up to 10 percent byweight of alkali metal oxides and said alkali metal oxides compriseNa₂O, Li₂O, K₂O and Ca₂O.
 6. Glass as defined in claim 1, furthercomprising, as refining agent, up to one percent by weight of at leastone of Sb₂O₃, SnO, NaCl, SO₄ ⁻ and F⁻.
 7. Lead-free optical glass with arefractive index n_(d) of 1.84≦n_(d)≦1.96 and an Abbé number ν_(d) of27≦ν_(d)≦36, and having a composition, in percent by weight based onoxide content, comprising: SiO₂  1 to 8, B₂O₃ 13 to 19.5, La₂O₃ 34 to50, MgO  0 to 6, CaO  0 to 6, BaO  0 to 6, ZnO  0 to 9, TiO₂  4 to 15,ZrO₂  0 to 11, Nb₂O₅  6 to 14.5,

wherein a sum total amount of MgO+CaO+BaO+ZnO is from 1 to
 10. 8. Glassas defined in claim 7, further comprising up to 10 percent by weight ofalkali metal oxides and said alkali metal oxides comprise Na₂O, Li₂O,K₂O and Ca₂O.
 9. Glass as defined in claim 7, further comprising, asrefining agent, up to one percent by weight of at least one of Sb₂O₃,SnO, NaCl, SO₄ ⁻ and F⁻.
 10. A glass for imaging, projection,telecommunications, optical communication technology or lasertechnology, said glass consisting of the glass as defined in claim 1.11. An optical element comprising the glass as defined in claim 1.